1. Field of the Invention
This invention relates generally to chemical mechanical planarization (CMP) systems, and more particularly, to systems having force applying air platens.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform Chemical Mechanical Planarization (CMP) operations, including polishing, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization.
In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
FIG. 1 illustrates an exemplary prior art CMP system 10. The CMP system 10 in FIG. 1 is a belt-type system, so designated because the preparation surface is an endless belt-type polishing pad 18 mounted on two drums 24 which drive the pad in a rotational motion as indicated by belt rotation directional arrows 26. A wafer 12 is mounted on a carrier 14. The carrier 14 is rotated in direction 16, which can be either clockwise or counterclockwise. The rotating wafer 12 is then applied against the polishing pad 18 with a force F to accomplish a CMP process. Some CMP processes require significant force F to be applied and monitored. A platen 22 is provided to stabilize the polishing pad 18 and to provide a support onto which to apply the wafer 12. The platen 22 is designed with an air bearing 23, which is designed to supply a constant flow of air during movement of the polishing pad 18. The constant flow of air therefore provides a consistent cushion over which the polishing pad 18 can traverse. To facilitate polishing, slurry 28 composed of an aqueous solution such as NH4OH or DI containing dispersed abrasive particles is introduced upstream of the wafer 12.
Typically, a load cell (LC) is integrated as part of the carrier 14 to enable monitoring of the pressure being applied to the wafer during processing. In practice, the carrier 14 is lowered onto the polishing pad 18 while the wafer is rotated in the direction 16. In addition to being lowered, the load cell (LC) is designed to provide pressure data to monitoring electronics. If more or less pressure is needed for a particular process, the spindle is instructed to make the pressure adjustment. Accordingly, not only is the spindle designed to move up and down, rotate at a particular rate, but also continuously adjust the force on the wafer (in the form of pressure) as transmitted by the carrier 14 to achieve the appropriate CMP parameters.
Because the carrier 14 is designed to place a force onto a moving polishing pad 18, frictional forces will build at the spindle so as to generate mechanical hysteresis. These frictional forces are known to reduce an actuator""s (which is designed to apply a force to the carrier 14) ability to maintain a constant force during small amplitude variations in carrier 14 vertical position during polishing. The challenge of maintaining a constant force during precision polishing operations therefore complicates the design of the carrier 14 and its accompanying electronics and controls. In some cases, even very expensive an complex controls are unable to ensure a uniform application of force since the carrier, which is measuring the forces, is continuously under frictional stress from the moving polishing pad 18.
In view of the foregoing, a need exists for a chemical mechanical planarization system that can provide a stable and accurate force to a substrate being planarized.
Broadly speaking, the present invention fills these needs by providing a chemical mechanical planarization system that has an adjustable platen. The adjustable platen is designed to apply a force to an underside of the polishing pad during operation, while the carrier is simply lowered into position over the polishing pad to achieve the appropriate planarization result. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a chemical mechanical planarization (CMP) system having a polishing pad, a wafer carrier, and an adjustable platen is disclosed. The adjustable platen includes a platen body and an air bearing that is integrated in the platen body for applying air pressure to an underside of the polishing pad. A set of bearings are connected to the platen body to enable movement of the platen body closer and further from the underside of the polishing pad. A load cell is connected to the platen body, and the load cell is configured to output a load signal that is indicative of a force being applied to the underside of the polishing pad. An air supply for applying air flow to the air bearing is also provided. The air flow is adjustable in response to changes in the force being applied to the underside of the polishing pad.
In another embodiment, an adjustable platen is disclosed. The adjustable platen includes a platen body having a top region and a bottom region. The platen body is oriented under a linear polishing pad. An air bearing is integrated with the platen body at the top region, and the air bearing is configured to apply an air pressure to an underside of the linear polishing pad. A set of bearings are connected to the bottom region of the platen body to enable controlled movement of the top region of the platen body closer or further from the underside of the linear polishing pad depending on the applied air pressure. The applied air pressure is configured to exert a controllable force to the underside of the linear polishing pad.
In yet another embodiment, another platen is disclosed. The platen includes a platen body having a top region and a bottom region. The platen body is positioned under a linear polishing pad of a chemical mechanical polishing (CMP) system, and the CMP system is designed to receive a wafer to be polished on a top surface of the linear polishing pad when positioned for processing by a spindle and carrier of the CMP system. An air bearing is coupled with the platen body at the top region, and the air bearing is configured to deliver an air flow to an underside of the linear polishing pad. A set of linear bearings are coupled to the bottom region of the platen body to enable controlled vertical movement of the platen body closer and further from the underside of the linear polishing pad. The vertical movement of the platen body is determined by the air flow, and the air flow is variable so as to set a desired force to the underside of the linear polishing pad.
In still another embodiment, a platen design is disclosed. The platen includes a platen body having a top region and a bottom region. The platen body is positioned under a linear polishing pad of a chemical mechanical polishing (CMP) system, and the CMP system is designed to receive a wafer to be polished on a top surface of the linear polishing pad when positioned for processing by a spindle and carrier of the CMP system. An air bearing is coupled with the platen body at the top region, and the air bearing is configured to deliver a fixed air flow to an underside of the linear polishing pad. A load cell for determining a force being applied to the underside of the linear polishing pad by the fixed air flow is integrated with the platen body. An actuator provided to vertically adjust the platen body closer and further from the underside of the linear polishing pad.
The advantages of the present invention are numerous. Most notably, by having the platen apply controlled forces from under the polishing pad, the spindle controlling the wafer carrier can be greatly simplified, and can eliminate the need for a splined spindle and complicated monitoring and compensating electronics. As is well known, the splines spindle is a mechanical device that allows for rotational motion about it""s long axis while allowing for translation about the same axis. Furthermore, the bearing(s) used to guide the platen are not affected by frictional forces, which eliminates the hystersis problems caused by side forces. Additionally, placing the load cell behind the platen as described in one of the possible system configurations would greatly reduce the complexity and cost of the load cell as well as dramatically increase reliability. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.